Power-tailored write-current method

ABSTRACT

A method of generating an encoded signal from a sequential stream of digital data, where the encoded signal has a non-power carrying null state and a power carrying active state with two opposing polarities. Logical one bits are distinguished from logical zero bits by inverting the encoded signal&#39;s polarity at the start of only the logical one bits. The encoded signal is set to the active state during a bit set-up period before, and held in the active state during a bit hold period after each polarity inversion. At other times the encoded signal is set to the null state. The method may include the addition of equalization pulses during strings of consecutive logical zero bits to keep the encoded signal from remaining in the null state for extended periods. Each equalization pulse may be preceded by an equalization set-up period and followed by an equalization hold period where the encoded signal is in the active state. In the preferred embodiment the set-up periods, hold periods, and equalization pulse periods are one-third the duration period of the logical bits.

TECHNICAL FIELD

The present invention is related to a field of encoding a sequentialstream of digital data.

BACKGROUND ART

Recording digital data on disks and tapes in accomplished by passing awrite current through a magnetic transducer to align the magnetic domainof the medium in one of two opposing directions. To achieve high datadensities on the medium, a non-return to zero inverting on the ones(NRZI) form of encoding is applied to the data stream to produce thewrite current. An NRZI encoded signal will maintain its present polarityfor each logical zero bit in the data stream, and invert its polarity atthe start of each logical one bit. The magnetic domain of the mediumfollows the write current by orienting in one direction when the writecurrent has a positive polarity, and in the opposite direction when thewrite current has a negative polarity.

NRZI encoding has a limitation in tape drive and disk drive applicationswhen presented with long strings of logical zero bits. During these longstrings, the write current remains at the same polarity. This causeslong regions in the data tracks to be magnetically oriented in the samedirection. Long regions of uniform magnetic orientation sometimes causea magneto-resistive read sensor to saturate, resulting in data errors.

The Full-Cell Write Equalization method was developed to prevent readsensor saturation. Full-Cell Write Equalization method briefly invertsthe write current polarity one or more times during long string ofzeros. These brief polarity inversions are called equalization pulses.For a string of N consecutive logical zero bits, the Full-Cell WriteEqualization method generates N−X equalization pulses. The value of X istypically equal to d, the minimum number of zeros between adjacent onesfor a (d,k) run length-limited modulation code. These equalizationpulses are typically centered in time during the writing of the logicalzero bits, starting with the first logical zero bit in the string. Otherencoding methods space the equalization pulses at equal intervalsbetween the NRZI transitions caused by logical one bits that lead andtrail the string of consecutive logical zero bits.

Another problem occurs where data stream encoding causes the writecurrent transitions at high frequencies. The net effect of these highfrequency transitions is to shift the apparent position of the leadingand trailing transitions as seen by the read sensor. These apparentshifts may sometimes cause problems in the read circuitry. The WritePre-Compensation method compensates for the apparent shifts seen byoffsetting the transition write positions in the opposite direction ofthe apparent shift. This method is well known in the field of diskdrives. Shifts in the apparent position of the data on magnetic mediumare sometimes caused by slow rise-times in the write transducers. TheWrite Pre-Emphasis method applies extra power at the start of atransition to produce a faster rise-time in the leading edge of thetransitions. While these two methods reduce the apparent shifts, theyrequire special circuitry to generate the complex write currents.

A common practice in the disk drive and tape drive industry is to readthe data from the magnetic medium as it is being written to verify thatthe correct data is being recorded. For space and alignment purposes,the read sensors and write transducers are usually co-located in thesame magnetic head. This close physical proximity, and the wiringconnecting the magnetic head to circuit cards, results in cross-talkbetween the write channel and the read channel. Signals in the writechannel are on the order of volts. Signals in the read channel are onlyon the order of millivolts. Consequently, any time power is applied tothe write channel it is picked up in the read channel as unwanted noise.

The Uniform Pulse-Write method partially solves the problem of writesignal noise in the read channel by reducing the amount of power in thewrite signal. This is accomplished by modulating the write current witha duty cycle so that part of the time the write current is not beingapplied to the write transducer. The frequency of the duty cycle issufficiently high so that the individual pulses overlap and blend in themagnetic medium. As a result, data written using the Uniform Pulse-Writemethod has the same read characteristics as data written using anon-pulsed method.

The Pulse-On-Transition method requires even less power than the UniformPulse-Write method. Pulse-On-Transition pulses the write current at thesame time that the Full-Cell Write Equalization method inverts the writecurrent's polarity. The Pulse-On-Transition method returns the writecurrent to zero at the completion of each pulse. Because some pulsesstart from a zero write current condition, data written using thismethod has different read characteristics than data written using theFull-Cell Write Equalization method making the two methods incompatible.Another reason the two methods can have different read characteristicsis that a portion of the magnetization in the write transducer mayswitch so slowly that the magnetization continues to increase as long asthe transducer is energized. Therefore, the magnetic state before andafter the polarity inversion may be very different for the two methods.

As recording densities increase, the signals from the read sensors willdecrease causing a lower signal to noise ratio. To maintain a reasonablesignal to noise ratio, the write current induced noise needs to bedecreased. A new encoding method is required that produces less writecurrent than the Full-Cell Write Equalization method. At the same time,the data written on magnetic medium using the new encoding method shouldhave the same read characteristics as data written using the Full-CellWrite Equalization method to maintain compatibility with existing diskdrives and tape drives.

DISCLOSURE OF INVENTION

The present invention is a method of generating an encoded signal from asequential stream of digital data, where the encoded signal has anon-power carrying null state and a power carrying active state with twoopposing polarities. Logical one bits are distinguished from logicalzero bits in the data stream by inverting the polarity of the encodedsignal at the start of only the logical one bits. The encoded signal isset to the active state during a bit set-up period before, and held inthe active state during a bit hold period after each polarity inversion.At other times the encoded signal is set to the null state. The methodmay include the addition of equalization pulses during strings ofconsecutive logical zero bits to keep the encoded signal from remainingin the null state for extended periods. Each equalization pulse ispreceded by an equalization set-up period, and followed by anequalization hold period during which the encoded signal is in theactive state. Where the equalization hold period of one equalizationpulse is contiguous with the equalization set-up period of the nextequalization pulse, the equalization set-up period may be eliminated.The equalization set-up period may also be eliminated for selectequalization pulses in a string of equalization pulses to balance themagnetic state of the write transducer. This approach is used for writetransducers that have slow magnetization reversal mechanisms. The slowmagnetization reversal mechanisms can result in an imbalance in themagnetization state when the write transducer is energized to onepolarity longer than the opposite polarity, averaged over time during ofthe string of equalization pulses. Finally, the equalization pulsepositions may be centered on the logical zero bits, spread uniformly intime between the logical one bit polarity inversions bounding thestring, or in other positions appropriate for the write transducer,recording medium and read sensor.

In an application where the encoded signal is a write current used witha magnetic transducer, the preferred method encodes N−1 equalizationpulses into each string of N consecutive logical zero bits. Theequalization set-up period, equalization hold period, and equalizationpulse periods are set at one-third the duration period of the logicalbits. Longer and shorter pulse periods may be used. Data recorded onmagnetic medium by a write current encoded by the preferred embodimenthas compatible read characteristics with data recorded using theFull-Cell Write Equalization method.

Accordingly, it is an object of the present invention to provide amethod for generating an encoded signal from a data stream, where theencoded signal has a non-power carrying state and a power carryingactive state with two opposing polarities.

Another object is to distinguish between logical one bits and logicalzero bits in the data stream by inverting the polarity of the encodedsignal at the start of each logical one bit, but not the logical zerobits.

Another object is to set the encoded signal to the active state during abit set-up period before, and during a bit hold period after invertingthe polarity at the start of each logical one bit. The encoded signalremains in the null state at other times.

Another object is to add equalization pulses during strings ofconsecutive logical zero bits in the data stream. The encoded signal isset to the active state during each equalization pulse, during anequalization set-up period before some or all of the equalizationpulses, and during an equalization hold period after each equalizationpulse.

Yet another object is to control the equalization set-up periods tobalance the time the encoded signal is active at each polarity duringstrings of equalization pulses.

These and other objects, features and advantages will be readilyapparent upon consideration of the following detailed description inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plot of an encoded signal produced by the present invention;

FIG. 2 is a bar chart of the probability of consecutive logical zero bitstrings in a random sample of data;

FIGS. 3A and 3B are plots of an encoded write current signal havingequalization pulses in the same positions as the Full-Cell WriteEqualization method, and the resulting read signal;

FIG. 4 is a plot of a write current signal from FIG. 3 having avariation on the encoding of the equalization pulses;

FIG. 5 is a plot of a write current signal having equalization pulsescentered between the leading and trailing logical one bit transitions;

FIG. 6 is a plot of the write current signal from FIG. 3 where the writecurrent has long set-up period and hold periods; and

FIG. 7 is a plot of the write current signal from FIG. 3 where the writecurrent has short set-up periods and hold periods.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 is a plot of an encoded signal 100 produced from a data stream102 using the method of the present invention. Each logical bit in datastream 102 has a value of logical one or logical zero, a bit startingtime, and a duration period. Using bit 2 and bit 3 as an example, thebit starting time of bit 2 is shown by line 104 and the bit startingtime for bit 3 is shown by line 106. The time separation between bitstarting time 104 and bit starting time 106 is the duration period ofbit 2. Bit 2 has a value 108 of logical zero. Encoded signal 100 has twostates. The first state is a null state 110 where the encoded signal 100carries no power. The second state is an active state where encodedsignal 100 carries power with either a positive polarity 112, or anegative polarity 114.

Encoded signal 100 distinguishes between the logical one bits andlogical zero bits in data stream 102 by inverting its polarity at thebit starting times of each logical one bit, and not the logical zerobits. The encoded signal 100 remains in the null state at all othertimes, including the duration of logical zero bits. Power carried by theencoded signal 100 is kept low by achieving the active state only duringa bit set-up period immediately before, and a bit hold periodimmediately after the bit starting times of logical one bits. Forexample, bit 3 begins at bit starting time 106. At a bit set-up period116 before bit starting time 106, the encoded signal 100 is set to theactive state with the positive polarity 112. At bit starting time 106,the polarity of the encoded signal 100 in inverted to the negativepolarity 114. During a bit hold period 118 after bit starting time 106,the encoded signal 100 remains in the active state with the negativepolarity 114. After the bit hold period 118, the encoded signal 100returns to the null state 110, shown in the figure as time 120. For theremainder of bit 3, all of bit 4, and most of bit 5, the encoded signalremains in the null state 110.

At a bit set-up period 122 before bit starting time 124 for bit 6, theencoded signal 100 is set to the active state. The polarity of encodedsignal 100 during bit set-up period 122 is the same negative polarity114 as during bit hold period 118. At bit starting time 124 the polarityof the encoded signal 100 is inverted again. The encoded signal 100remains in the active state during bit hold period 126, and thentransitions to the null state 110. This method is repeated for eachlogical one bit in data stream 102. During a bit set-up period beforethe bit starting time of each logical one bit, the encoded signal 100 isset to active with the same polarity it had last time it was active. Thepolarity is inverted at the bit starting time. After a bit hold periodthe encoded signal 100 is returned to the null state 110. Note that thefirst logical one bit in the data stream 102 has no prior active statepolarity against which to reference. The method accounts for this byrequiring that a starting polarity be defined as part of an initialcondition. Either a positive polarity 112 or a negative polarity 114 maybe used. In the example shown in FIG. 1, the negative polarity 114 isused as the initial condition.

Encoded signal 100 will be used to deliver power to a transducer or loadin practical applications. Since the characteristics of varioustransducers and loads vary, the ideal bit set-up period and bit holdperiod of the encoded signal will vary from application to application.The bit set-up period will be longer than the bit hold period in someapplications, shorter in others, and sometime the bit set-up period andbit hold period will have the same duration. The preferred embodimentprovides symmetry among the bit set-up period, bit hold period and theminimum period the encoded signal 100 spends in the null state 110. Anexample of the minimum null state period is provided by bit 10 and bit11. Since both bits have logical one values, the encoded signal 100 isactive during the bit hold period 128 after the bit starting time 130 ofbit 10, and active again during the bit set-up period 132 before the bitstarting time 134 of bit 11. The time between bit hold period 128 andbit set-up period 132, shown in the figure as period 136, is the minimumperiod during which the encoded signal 100 is in the null state 110. Forthe hold period 128 to be the same as the bit set-up period 132 and thenull state period 136, each is one-third of the duration period of bit10.

Applying the method to the write current in tape drives and disk drives,the issue of consecutive logical zeros strings must be taken intoaccount. When the write current is in an active state, the magnetictransducer orients the magnetic domain of the medium over a length ofapproximately three bits. This means that the bit hold period after eachlogical one bit starting time writes over the remainder of the logicalone bit and up to two following logical zero bits. FIG. 2 shows theprobability of having string of one to seven consecutive logical zerobits in 1,048,576 bytes of random data. Approximately 38% of the logicalzero bits are bounded by logical one bits. The percentage decrease toless than a 1% probability of finding seven consecutive logical zerobits. The average string of consecutive logical zero bits is 2.33. Thepreferred embodiment accounts for this condition by adding equalizationpulses to the encoded signal 100 during strings of two or moreconsecutive logical zero bits.

FIG. 3 is a plot of a write current 300 used by a typical magnetictransducer to write data on a magnetic medium, and the resulting readsignal 302 produced by a typical magneto-resistive sensor. Write current300 includes equalization pulses at the same positions in time as wouldbe produced using the Full-Cell Write Equalization method. In thepreferred embodiment, N−1 equalization pulses are encoded for eachstring of N consecutive logical zero bits, where N is at least 2. Ingeneral, N−X equalization pulses may be encoded for each string of Nconsecutive logical zero bits, where X is an integer equal to or lessthan N.

Encoding of the equalization pulses is similar to the encoding of thelogical one bits. During an equalization set-up period before eachequalization pulse starting time, the write current 300 is set to theactive state with the same polarity as during the prior hold period.Write current 300 reverses polarity once at the equalization pulsestarting time, and again at the equalization pulse ending time(returning to the same polarity as during the equalization set-up periodbefore the equalization pulse). Finally, after an equalization holdperiod, the write current 300 is set to the null state. In a variation,some equalization pulses may be encoded without the equalization set-upperiod. For these equalization pulses, the write current 300 is setactive at the equalization pulse starting time with the oppositepolarity as during the prior hold period.

The relationship between equalization set-up periods, equalization holdperiods and the duration between the equalization pulse starting timeand equalization pulse ending time will vary from application toapplication. In some applications, the equalization periods andequalization pulse durations will be symmetrical, in others they willnot. Likewise, the relationship between the bit set-up period,equalization set-up period, bit hold period and equalization hold periodwill vary from application to application. The bit periods may belonger, equal to, or shorter than the equalization periods. In thepreferred embodiment, the set-up periods, hold periods and equalizationpulse durations are all the same at one-third of the bit duration.

Positioning of the equalization pulses in the preferred embodiment maybe designed to center them between the bit starting times of the logicalone bits leading and trailing the string. For a string of N consecutivelogical zero bits, one equalization pulse is centered on each of thefirst N−1 bits. The last (N^(th)) bit in the string does not have anequalization pulse. This is done so that the period between the polarityinversion, due to the logical one bit leading the string, and the centerof the first equalization pulse is the same as the period between thecenter of the last equalization pulse and the polarity inversion due tothe logical one bit trailing the string.

Bit 7, bit 8 and bit 9 are an example of three consecutive logical zerobits (N=3). The method encodes two (N−1) equalization pulses 304 and 306during this string. Equalization pulse 304 is preceded by an activeequalization set-up period 308, and followed by an equalization holdperiod 310. Equalization pulse 306 is preceded by an equalization set-upperiod 312 and followed by equalization hold period 314. Bit 9 does nothave an equalization pulse. As a result, the period between the polarityinversion for bit 6 and the center of the equalization pulse 304, shownas period 316, matches the period between the polarity inversion for bit10 and the center of the equalization pulse 306, shown as period 318.

Setting the equalization pulse periods to the preferred minimumone-third the duration period allows the equalization set-up period,equalization pulse duration, and equalization hold period to occurwithin one bit duration. When two equalization pulses appearback-to-back, as in bit 7 and bit 8, the equalization hold period 310 ofone equalization pulse is contiguous with the equalization set-up period312 of the next. The encoded signal does not return to the null state atthe end of the equalization hold period 310. To account for thissituation, the encoding method must look ahead to the next bit in thedata stream 102. If the next set-up period begins during the currenthold period, then the encoded signal is not returned to the null stateat the end of the hold period. If the next set-up period will not occurduring the current hold period, then the encoded signal is returned tothe null state after the hold period has ended.

Data written on a magnetic medium by write current 300 produces the readsignal 302. Read signal 302 reaches peak values at times 320-328. Thesecorrespond to the bit starting times for bits 1, 6, 10, 11 and 12. Byincluding the N−1 equalization pulses, their associated equalizationset-up periods, and equalization hold periods in the write current 300,the read signal 302 is the same as if the data had been written usingthe Full-Cell Write Equalization method. This allows a tape drive ordisk drive implementing the present invention to swap magnetic mediumwith a tape drive or disk drive implementing the Full-Cell WriteEqualization method.

FIG. 4 is an alternative encoding method that reduces the write currentpower required during consecutive equalization pulses. In thisembodiment, where the equalization hold period of one equalization pulseis contiguous with the equalization set-up period of the nextequalization pulse, the equalization set-up period of the nextequalization pulse is eliminated. Data stream 102 shows an example of afive consecutive logical zero bit string in bit 13 through bit 17. Fourequalization pulses 400-406 are generated centered on bit 13 through bit16. The first equalization pulse 400 is preceded by an activeequalization set-up period 408 followed by an equalization hold period410, similar to equalization pulse 304 in FIG. 3. The second throughforth equalization pulses 402-406 are generated without the equalizationset-up periods. At the end of equalization hold period 410 the writecurrent 300 is set to the null state. Write current 300 is set to theactive state again with the opposite polarity as during equalizationhold period 410 at the equalization pulse starting time 412 forequalization pulse 402. At the equalization pulse ending time, thepolarity of write current 300 is inverted to the negative polarity whereit remains active during equalization hold period 414. Write current 300is set to the null state at the end of equalization hold period 414. Theencoding for the second equalization pulse 402 is repeated for the thirdand forth equalization pulses 404 and 406. For a string of N consecutivelogical zero bits there are N−2 consecutive equalization pulses encodedwithout the equalization set-up period. Using an average of N=2.33consecutive logical zero bits, this method reduces the average time thatthe write current is active during equalization pulses by 11% ascompared with the encoding method of FIG. 3.

Additional variations for encoding strings of equalization pulses may beemployed to mitigate the effects of slow magnetization reversalmechanisms in the write transducers. The slow magnetization reversalmechanisms can result in an imbalance in the transducer's magnetizationstate when it is energized to one polarity for a greater average timethan it is energized to the opposite polarity. To account for thisbehavior, strings of equalization pulses are encoded so that some, butnot all of the equalization pulses are preceded by the equalizationset-up period. One variation would be to precede only the first and thelast equalization pulse in a long string of equalization pulses with theequalization set-up period. For example, in FIG. 4 equalization pulses400 and 406 are preceded by an equalization set-up period whileequalization pulses 402 and 404 are not. Another variation would be toprecede every other equalization pulse with the equalization set-upperiod. Here, equalization pulses 400 and 404 are preceded by theequalization set-up period, while equalization pulses 402 and 406 arenot. Many variations of this approach may be applied where theequalization set-up periods of select equalization pulses are of zeroduration.

FIG. 5 is another method for producing the equalization pulses. In thismethod the period between equalization pulse centers, and the periodsbetween the end equalization pulse centers and the logical one bitpolarity inversions leading and trailing the string are the same. For astring of N consecutive logical zero bits and N−X equalization pulses,the uniform spacing is (N+1)/(N−X+1) duration periods. Write current 300shows two examples of uniform equalization pulse spacing for X=1. In thefirst example, bit 7 through bit 9 form a three consecutive logical zerobit string (N=3). The uniform spacing 500 for this string is four-thirdsthe duration period. The net effect is that equalization pulse 304 isshifted earlier in time in FIG. 5 than in FIG. 3, while equalizationpulse 306 is shifted later in time. Note that using this encoding methodthe equalization hold period 310 is not contiguous with the equalizationset-up period 312 as they are in FIG. 3. In the second example, bits 13through bit 17 form a string of five consecutive logical zero bits. Theuniform spacing 502 for this string is six-fifths the duration period.

Other variations of the equalization pulses can be employed within thescope of the present invention. For example, the encoding may use onlyN−2 equalization pulses in a string of N consecutive logical zero bits.Overall, N−X equalization pulses may be inserted into a string of Nconsecutive logical zero bit in a variety of positions with respect tothe logical one and logical zero bits. The integer X and the positioningof the equalization pulses are determined by the characteristics of themedium recording the encoded data, the read sensor reading the data, andthe minimum number of zeros allowed between consecutive ones by amodulation code.

FIG. 6 shows another embodiment of the present invention that accountsfor slow rise times in the write transducer. In this encoding method thewidth of the set-up periods, hold periods, and equalization pulesperiods are longer than the preferred one-third duration period. Widerwrite current pulses provide more time for the write transducer toorient the magnetic domain in the magnetic medium. The tradeoff is anoverall increase in the write current power, and thus in the noisecoupled to the read signal 302 (not shown). As shown in FIG. 6, writecurrent 300 has set-up periods, hold periods and equalization pulseperiods of one-half the duration period. This method causes some holdperiods to merge with the preceding set-up periods. For example, the bithold period 128 following the bit starting time 134 of bit 10 iscontiguous with the bit set-up period 132 preceding bit starting time134 of bit 11. In another example, the equalization hold period 310following equalization pulse 304 overlaps with the equalization set-upperiod 312 preceding equalization pulse 306.

FIG. 7 is an embodiment for use where the rise time of the writetransducer and medium are fast. The set-up periods, hold periods andequalization pulse widths in this example at one-quarter the durationperiod. Write current 300 is in the active state for short durations oftime, so the power supplied to the write transducer is reduced ascomparted with the preferred embodiment. The magnetic medium used withthis embodiment must be able to orient its magnetic domain quickly tocapture the data. One feature of this embodiment to note is that theequalization hold period and equalization set-up period of consecutiveequalization pulses are not contiguous. See for example equalizationhold period 310 and equalization set-up period 312 in bit 7 and bit 8.Other short pulse periods may be used within the scope of the presentinvention. The practical limitation for short pulse periods will beeither the response of the transducer using the encoded signal, or themedium recording the data.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

What is claimed is:
 1. A method for generating an encoded signal from adata stream, where the encoded signal has a null state and an activestate with opposing polarities, and wherein the data stream has logicalone bits and logical zero bits, each logical one bit and logical zerobit having a bit starting time, the method comprising: establishing astarting polarity of the opposing polarities; setting the encoded signalto the active state at approximately a bit set-up period prior to thebit starting time of each logical one bit in the data stream; invertingthe polarity of the encoded signal at approximately the bit startingtime of each logical one bit in the data stream; and setting the encodedsignal to the null state at approximately a bit hold period after thebit starting time of each logical one bit where no subsequent setting ofthe encoded signal to the active state occurs during the bit holdperiod.
 2. The method of claim 1 wherein the encoded signal remains inthe null state in response to each logical zero bit.
 3. The method ofclaim 1 further comprising: detecting each string of at least oneconsecutive logical zero bit in the data stream; defining at least oneequalization pulse during each string of at least one consecutivelogical zero bit in response to detecting each string of at least oneconsecutive logical zero bit in the data stream, where each equalizationpulse of the at least one equalization pulse has an equalization pulsestarting time and an equalization pulse ending time; setting the encodedsignal to the active state at approximately an equalization set-upperiod prior to each equalization pulse starting time; inverting thepolarity of the encoded signal at approximately each equalization pulsestarting time; inverting the polarity of the encoded signal atapproximately each equalization pulse ending time; and setting theencoded signal to the null state at approximately an equalization holdperiod after each equalization pulse ending time where no subsequentsetting of the encoded signal to the active state occurs during theequalization hold period.
 4. The method of claim 3 further comprisingsetting the equalization set-up period of at least one selectedequalization pulse to approximately zero duration in response todefining at least one equalization pulse during each string of at leastone consecutive logical zero bit.
 5. The method of claim 4 wherein theat least one equalization pulse is at least two equalization pulses, andthe at least one selected equalization pulse is each equalization pulseexcept the first equalization pulse of the at least two equalizationpulses.
 6. The method of claim 4 wherein the at least one equalizationpulse is at least two equalization pulses, and the at least one selectedequalization pulse is every other equalization pulse of the at least twoequalization pulses.
 7. The method of claim 4 wherein the at least oneequalization pulse is at least three equalization pulses, and the atleast one selected equalization pulse is each equalization pulse exceptthe first equalization pulse and the last equalization pulse of the atleast three equalization pulses.
 8. The method of claim 1 furthercomprising: detecting each string having a multiple number ofconsecutive logical zero bits in the data stream; defining a number ofequalization pulses during each string having the multiple number ofconsecutive logical zero bits in response to detecting each stringhaving the multiple number of consecutive logical zero bits, wherein thenumber of equalization pulses ranges from one to less than the multiplenumber of consecutive logical zero bits respectively, and eachequalization pulse of the number of equalization pulses has anequalization pulse starting time and an equalization pulse ending time;setting the encoded signal to the active state at approximately anequalization set-up period prior to each equalization pulse startingtime; inverting the polarity of the encoded signal at approximately eachequalization pulse starting time; inverting the polarity of the encodedsignal at approximately each equalization pulse ending time; and settingthe encoded signal to the null state at approximately an equalizationhold period after each equalization pulse ending time where nosubsequent setting of the encoded signal to the active state occursduring the equalization hold period.
 9. The method of claim 8 where thenumber of equalization pulses is one less than the multiple number ofconsecutive logical zero bits respectively, and each equalization pulseis approximately centered in time on each logical zero bit, except thelast logical zero bit in each string having the multiple number ofconsecutive logical zero bits.
 10. The method of claim 9 wherein eachlogical bit has a duration period, and wherein the bit set-up period,the bit hold period, the equalization set-up period and the equalizationhold period are approximately one-third of the duration period, and foreach equalization pulse the equalization pulse ending time occursapproximately one-third of the duration period after the equalizationpulse starting time respectively.
 11. The method of claim 1 wherein theencoded signal is a write current for use with a magnetic writetransducer.